1. Field of the Invention
The present invention relates to an integrated transformer, and more particularly to an integrated transformer with a stack structure.
2. Description of the Related Art
For integrated circuits applied in wireless communication, transformers convert impedance among different signals. In order to effectively reduce circuit interference resulting from common-mode noises, more and more circuits adopt the design of differential signal pairs. Accordingly, transformers must transform single-ended unbalance signals into differential balance signals. One of these transformers is the balance-to-unbalance (BALUN) transformer.
FIG. 1 is a schematic drawing showing an equivalent circuit of a BALUN transformer. Referring to FIG. 1, the BALUN transformer 100 comprises a primary side P and a secondary side S. Wherein, the first terminal 11 of the primary side P of the BALUN transformer 100 receives/outputs unbalance signals, and the second terminal 13 is grounded. In addition, the secondary side S comprises a first terminal 15, a second terminal 17, and a center tap 19. Wherein, the center tap 19 is coupled to a reference voltage which is generally grounded. The first terminal 15 and the second terminal 17 of the secondary side S outputs/receives inversed balance signals, respectively.
FIG. 2 is a configuration showing a conventional BALUN transformer. Referring to FIG. 2, conductive lines 21 and 23 wind like a spiral in the BALUN transformer 200. Wherein, two terminals of the conductive line 21 are two terminals of the primary side P, receiving/outputting unbalance signals, respectively. Two terminals of the conductive line 23 are two terminals of the secondary side S, outputting/receiving balance signals, respectively. The disadvantage of the BALUN transformer 200 is that the location of the center tap 25 can only be determined after electrical performance of winding is measured.
In order to solve the issue in FIG. 2, U.S. Pat. No. 3,904,911 discloses several BALUN transformers. In these BALUN transformers disclosed in U.S. Pat. No. 3,904,911, the winding conductive line is only one circle and is not practical.
FIG. 3A is a configuration showing another conventional BALUN transformer. Referring to FIG. 3A, the integrated circuit comprises symmetric windings and the location of the center tap CT can be easily determined. This structure, however, has an asymmetric pattern between the winding of the primary side P and the winding of the secondary side S.
FIG. 3B is a configuration showing another conventional BALUN transformer. Referring to FIG. 3B, it is a BALUN transformer disclosed in U.K. Patent No. 8,800,115. Though the BALUN transformer disclosed in FIG. 3B can resolve the issue in FIG. 3A, the area required for the transformer is relatively larger. As a result, the area of the integrated circuit also increases.
FIG. 4A is a top view of a conventional BALUN transformer. FIG. 4B is a cross sectional view of the BALUN transformer of FIG. 4A along 4K–4K′. In order to solve the issue for the large area required in FIG. 3B, a BALUN transformer with a stack structure is disclosed as shown in FIGS. 4A and 4B.
It is known from FIGS. 4A and 4B, the conventional BALUN transformer with the stack structure comprises a top winding 41 and the bottom winding 43, which wind over the first surface and the second surface of the dielectric layer 45, respectively. Wherein, two terminals of the top winding 41 are two terminals of the primary side P of the BALUN transformer 400. Similarly, the two terminals of the bottom winding 43 are two terminals of the secondary side S of the BALUN transformer 400. With the stack structure, the area required for the BALUN transformer 400 can be reduced.
The BALUN transformer 400 still has some disadvantages. In FIG. 4A, due to the asymmetric pattern between the top winding 41 and the bottom winding 43, the location of the center tap is hard to determine. In addition, the distance from the top winding 41 to the substrate 47 is different from the distance from the bottom winding 43 to the substrate 47. As a result, the parasitic capacitance on the primary side and the secondary sides are different. Therefore, electrical characteristics of the BALUN transformer 400 are hard to control.